1. Field of the Invention
The present invention relates to the field of optics and, in particular, to laser optics, as used in optical analyzers, such as flow cytometers.
2. Description of Related Art
Particle analyzers, such as flow and scanning cytometers, are well known analytical tools that enable the characterization of particles on the basis of optical parameters such as light scatter and fluorescence. In a flow cytometer, for example, particles, such as molecules, analyte-bound beads, or individual cells, in a fluid suspension are passed by a detection region in which the particles are exposed to an excitation light, typically from one or more lasers, and the light scattering and fluorescence properties of the particles are measured. Particles or components thereof typically are labeled with fluorescent dyes to facilitate detection, and a multiplicity of different particles or components may be simultaneously detected by using spectrally distinct fluorescent dyes to label the different particles or components. Typically, detection is carried out using a multiplicity of photodetectors, one for each distinct dye to be detected. Both flow and scanning cytometers are commercially available from, for example, BD Biosciences (San Jose, Calif.). A description of flow cytometers is provided in Shapiro, 2003, Practical Flow Cytometry, 4th ed. (John Wiley and Sons, Inc. Hoboken, N.J.), and in the references cited therein, all incorporated herein by reference.
In a typical flow cytometer, the excitation light from a laser or other source is focused onto a focal spot to illuminate the core stream (the fluid stream containing the particles to be analyzed). Accurate focusing of the excitation light beam on the core stream is important for optimizing focal spot intensity and, thus, fluorescence sensitivity. Optimal performance is compromised if the focused light beam is not properly adjusted on the core stream, and flow cytometers typically include one or more devices for adjusting the positioning of the focused light beam on the core stream. Because a typical flow cytometer is designed to analyze biological cells or particles that are few microns in size, the precision of the light beam adjustment also needs to be in the micron range, thus requiring high resolution mechanical displacement devices. Conventional positioning methods typically employ expensive differential micrometers to position the light source itself or optical elements, such as minors or prisms.
U.S. Pat. No. 4,989,977 describes a device for the accurate adjustment of the focused excitation beam on the core stream. Repositioning of the focal point is achieved using a transparent glass plate located between the focusing lens and the core stream. The glass plate, when positioned at an angle to the beam path, displaces the focal point by refracting the beam. In a multi-laser instrument, the glass plate typically is positioned between the focusing lens and the core stream, and all beams in a multi-laser instrument are passed through the single plate.
U.S. Patent Publication no. US 2009/0073579 describes another device for the accurate adjustment of the focused excitation beam on the core stream. A movable beam-adjusting lens having a long focal length lens is positioned in the optical path upstream of the focusing lens. Repositioning of the focal point is achieved by adjusting the upstream movable beam-adjusting lens. The sensitivity of the focus spot positioning to adjustments of the beam-adjusting lens position depends on the focal length of the beam-adjusting lens; the longer the focal length of the beam-adjusting lens, the less sensitive the position of the focus spot is to changes in the position of the beam-adjusting lens. The decreased sensitivity allows the use of less expensive, less precise lens positioning mechanisms, such as simple screw-type positioning systems, to obtain precise positioning control over the beam focus spot.